a. Field of the Invention
This invention relates to a process for preparing cyclohexanonecarboxylic acid compounds.
b. Description of the Prior Art
Cyclohexanonecarboxylic acid compounds, such as cyclohexanone-4-carboxylic acid and the like, are known to be useful as perfume enhancers and the like (see U.S. Pat. No. 4,537,704). Moreover, these compounds are also useful as starting materials for the synthesis of polymers and as intermediates for the synthesis of liquid crystals. A large number of processes for the synthesis of cyclohexanonecarboxylic acid compounds have been proposed. Among them, the process which comprises hydrogenating a hydroxybenzoic acid compound and selectively oxidizing the resulting cyclohexanolcarboxylic acid compound with the aid of an oxidizing agent such as a chromium compound or the like [as described in J. Am. Chem. Soc., 86(15), 3068-72 (1962)] is considered to be useful for inductrial purposes.
On the other hand, an attempt has been made to prepare cyclohexanone-4-carboxylic acid methyl or ethyl ester by direct and selective hydrogenation of methyl or ethyl p-hydroxybenzoate (see U.S. Pat. No. 4,537,704).
Of these two processess, the former one in which a cyclohexanonecarboxylic acid compound is prepared by way of a cyclohexanolcarboxylic acid compound involves two process steps. Moreover, the hydrogenation step yields such by-products as the dehydroxylated product, lactonized product and the like, and the oxidation step fails to give a satisfactory yield. Furthermore, it is necessary to recover and regenerate the oxidizing agent. Thus, it is hard to say that this process is highly economical.
On the other hand, the process of U.S. Pat. No. 4,537,704 based on the direct hydrogenation of methyl or ethyl p-hydroxybenzoate is hardly applicable to high-melting compounds such as p-hydroxybenzoic acid, because the reaction is carried out in the absence of solvent. Moreover, where methyl p-hydroxybenzoate, for example, is hydrogenated, the over-hydrogenated product (i.e., cyclohexanol-4-carboxylic acid methyl ester) is formed as a by-product in an amount of 30% or more. Thus, the yield of cyclohexanone-4-carboxylic acid methyl ester is less than satisfactory.